Sunday, 10 June 2012

CNX2 WB Correction Confusions

[ These are notes for my WB hacking with CNX2 - it's not meant to be a WB tutorial ]

Still trying to get to grips with CNX2 white balancing for skin tones. I understand that skin tones should result in R>G>B, with a ratio of [ 1 | 0.85 | 0.75 ] being 'reasonable'.

red top | blue bottom | neutral third left

However, most skin tone tutorials refer to corrections in CYMK but CNX2 gives us RAW development corrections with red-blue values only, although it does give us colour balance corrections with CYMK in the adjustment section.

What are the problems with performing development WB corrections with red and blue only?

RGB color wheel @ wikipedia

A couple of statements up front

From wikipedia entry on RGB color:
When one of the components has the strongest intensity, the color is a hue near this primary color (reddish, greenish, or bluish), and when two components have the same strongest intensity, then the color is a hue of a secondary color (a shade of cyan, magenta or yellow). A secondary color is formed by the sum of two primary colors of equal intensity:
  • cyan is green + blue
  • magenta is red + blue
  • yellow is red + green
Every secondary color is the complement of one primary color; when a primary and its complementary secondary color are added together, the result is white: cyan complements red, magenta complements green, and yellow complements blue.

And some observations:
  • A increase of blue, green decreases
  • A decrease of blue, green increases
  • A increase of red, green decreases followed by blue
  • A decrease of red, blue increases followed by green

CNX2 WB correction examples

Each correction is from the same base corrected image - a WhiBal corrected RAW file; a watch point was selected which showed a neutral RGB value (190/190/190 in this case) so we can observe the shift as we make our corrections. Percentage adjustments were then made.

Note the corresponding changes to the image and the neutral point as we shift red or blue. Also, note the changes on histogram.

All WB corrections are via CNX2's Develop -> Camera Settings -> White Balance section.

Single (red or blue) Corrections

RED+25% (rose)+10%neutral-10%-25%-50% (cyan)
BLUE+25% (violet)+10%neutral-10%-25%-50% (??)

Double (red and blue) Corrections

red +25% blue -25% (orange)red +25% blue +25% (magenta)
red -25% blue +25% (blue)red -50% blue -50% (green)

And the Real World

CNX2's Color Balance adjustment tool provides the standard adjustments: red - cyan | green - magenta | blue - yellow.

Of course, color balance adjustments can be made by the inbuilt tool. But if you were perverse, we are able to make these adjustments via the WB red/blue values directly. For example, a common adjustment is to remove a green cast which involves adding more magenta - from above we know this involves an increase of red and blue.

The watch point#2 reports an RGB average of 190 with R:232 G:176 B:156. This is R:1 G:0.75 B:67

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